In ultrasonic internal examination, for example, in case of radial scan type ultrasonic internal examination using mechanical radial scanning, an ultrasonic vibratory element is usually accommodated in a casing which contains an ultrasonic transmissive medium in a sealed state. As the ultrasonic vibratory element is turned manually or mechanically through a drive means, ultrasonic energy is directed into an intracavitary wall of interest while receiving return echo signals from the wall tissues and processing the received signals at an ultrasonic image observation terminal to display on a monitor an ultrasonic image which gives information concerning a cross-sectional area of the wall tissues.
In order to improve the picture quality of the abovementioned sectional ultrasonic image, it is necessary to narrow the ultrasonic beam to be directed into a body. This can be achieved to a certain extent, for example, by employing an aperture of suitable construction for the ultrasonic vibratory element. However, it is difficult to converge the ultrasonic beam to a necessary degree simply through revision of the aperture construction of the ultrasonic vibratory member, which requires troublesome machining operations and would result in a very expensive ultrasonic vibratory element.
In this connection, there has been developed an ultrasonic probe construction, in which the ultrasonic transmissive medium sealed in the casing of the vibratory element is used as an acoustic lens based on the difference in acoustic impedance of the ultrasonic transmissive medium from the casing and intracavitary wall tissues to be examined.
In utilizing the ultrasonic transmissive medium in the casing as an acoustic lens as mentioned above, it may be conceivable to employ a casing of hollow spherical shape for the ultrasonic vibratory element. However, a problem which arises in the fabrication of casings of such a hollow spherical shape is that the machining of the inner wall surfaces is extremely difficult. Besides, if incorporated into an ultrasonic probe to be inserted into human body for ultrasonic examination, the tip end of the probe would be bulged out in spherical shape, making it difficult to insert the probe into a narrow or constricted intracavitary portion and at the same time degrading the operationability of the probe, for example, by increasing the resistance to the efforts of withdrawal.
In case of an endoscope, typical of the instruments widely used for internal examination and diagnosis, its insert portion is normally of a cylindrical shape having a uniform diameter substantially over the entire length thereof. This shape has been selected in consideration of various conditions including operability at the time of insertion etc. For instance, the fore end of the insert portion is formed so as to have a smooth and uniform outer configuration almost free of surface irregularities to ensure smooth passage through constricted portions which might exist in the path of insertion of the endoscope. The ultrasonic internal examination system, which is designed to be inserted into a human body similarly to the endoscope, is preferred to have an insert portion of a cylindrical shape having a uniform diameter substantially over its entire length. In addition, a spherically shaped tip end is disadvantageous in consideration of the necessity for fitting a balloon at the tip end of the insert portion for the purpose of standoff, which is required by the ultrasonic examination system itself. Further, in a case where the ultrasonic examination system is used as an ultrasonic endoscope which combines the functions of the endoscope with an ultrasonic examination system, a bulged portion at the tip end of the ultrasonic probe should be avoided from the standpoint of securing a sufficient field of view and preventing reflections of illuminating light.
For the reasons stated above, existing ultrasonic internal examination systems of the type to be inserted into an intracavitary or other internal portion invariably have an ultrasonic vibratory element accommodated in a cylindrical rigid tip portion. An ultrasonic probe with such a cylindrical tip end has a circular shape in section in a direction perpendicular to its axis and thus could be arranged to have the function of acoustic lens in the axial direction. On the other hand, it has a flat shape in section in the axial or longitudinal direction, which practically would not lead to any useful lens effects. Namely, a radial ultrasonic beam pattern directed from the ultrasonic vibratory element can only be converged into an undesirably flattened elliptical shape.